Snowmobiles have increased in power and speed. With these increases have come more aggressive skis and tracks, along with extended suspension travel. Increased vertical travel results in more lateral ski movement upon compression of the suspension. Such lateral movement makes the snowmobile feel unstable. It may feel as if it will dart to one side or the other. For example if a bump is encountered on one ski the outward movement of the ski that occurs with the upward suspension travel will tend to push the snowmobile to the opposite direction. The steering may also jerk. Furthermore, the ski and its skag tend to hunt grooves and ruts, giving a feeling of uneasiness to the rider as the skis pull the steering and snowmobile side to side.
Lateral ski movement with suspension action is due to the common suspension arms being pivotally secured to the snowmobile chassis. As the arms pivot upwardly relative to the chassis, they also swing outwardly and/or inwardly swing through an arcuate path. The arms are initially angled downwardly from the chassis, before suspension compression. As the suspension is compressed the arms initially swing up and laterally out. If the arms travel past horizontal the ends come slightly back in towards the chassis. Such lateral movement may diminish the control of the snowmobile and the overall smoothness of the ride.
Beyond these more obvious disadvantages, the suspension is somewhat less able to freely travel as it is bound by the bite of the skis into the snow. In order for the suspension to travel upwardly relative to the chassis, it must overcome the force keeping the ski tracking. It must push one or both skis to the side. This binds the suspension to some extent from absorbing vertical impacts. Generally the ski with the better bite at the time a terrain feature is encountered will track while the other ski will slide to the side. This is attenuated somewhat by the mass of the snowmobile. However, the rider may not be able to predict the lateral action in the split second required as the suspension is compressed.
The problem is amplified at high speed. Quick, unexpected lateral movements of the snowmobile that are induced by the front suspension reacting to undulations in the terrain can be dangerous. The rider may find it very tiring having to be extremely vigilant is watching for every little bump that he or she may encounter. If visibility is low, such as at night, or a passenger is aboard the driver must significantly decrease speed to travel safely. The passenger is not typically as active or able to watch for undulations in terrain. The passenger is likely to be surprised by the lateral movements.
The problem has been recognized, although it has been amplified with increased suspension travel in the past several years. U.S. Pat. No. 5,029,664, incorporated herein by reference, depicts a four-bar linkage system that tilts the lower end of the steering spindle inwardly as the suspension is compressed such that the upward path of a point on the bottom of the ski is more vertical. Such a system may improve stability in some situations, but becomes more difficult to carry out with increases in suspension travel. Change in the side-tilt angle of the ski that would result from tilting the spindle as the suspension is compressed may cause the ski to dart to one side or the other, especially with the more aggressive skis available. Many skis have skags and/or sidecuts that help turn the ski as it is tilted. Thus, excessive tilting may tend to exacerbate the problem.
One approach, depicted in U.S. Pat. No. 4,489,954, employs a strut-type suspension for controlling movement of a snowmobile ski relative to the chassis. This type of system has found extensive usage in automobiles, but some shortcomings have become apparent when it is used on snowmobiles due to rough terrain and severe conditions in which snowmobiles are often used. One problem being the cost of replacement struts when they are damaged. Use of the system in long travel suspensions is also difficult. The system also becomes more prone to damage as its size is increased for increased travel. Furthermore, friction and lateral alignment may be a problem, especially when cornering.
Thus an unmet need exists for a suspension and tracking system that overcomes the disadvantages of lateral suspension movement without suspension binding, excessive expense, or ski tilting problems.